Atomic force microscopy is based upon the principle of sensing the forces between a sharp stylus or tip and the surface to be investigated. The interatomic forces induce the displacement of the tip mounted on the free end of a cantilever.
As described by Binnig et al., "Atomic Force Microscope", Phys. Rev. Lett., Vol. 56, No. 9, Mar. 3, 1986, pp. 930-933, a sharply-pointed tip is attached to the free end of a flexible spring-like cantilever to scan the profile of a surface to be investigated. The attractive or repulsive forces occurring between the atoms at the apex of the tip and those of the surface result in tiny deflections of the cantilever. In its original implementation, a tunneling junction was used to detect the motion of the tip attached to an electrically-conductive cantilever. An electrically-conductive tunnel tip is disposed within the tunnel distance from the back of the cantilever, and the variations of the tunneling current are indicative of the cantilever deflection. The forces occurring between the tip and the surface under investigation are determined from the measured cantilever deflection and the characteristics of the cantilever.
The principle of atomic force microscopy has been extended to the measurement of magnetic, electrostatic, and frictional forces, with the tip operating in either contact or near-contact with the surface of the sample. Magnetic force microscopy using a magnetized iron tip is described by Martin et al., "High-resolution Magnetic Imaging of Domains in TbFe by Force Microscopy", Appl. Phys. Lett., Vol. 52, No. 3, Jan. 18, 1988, pp. 244-246. Electrostatic force microscopy is described by Terris et al., "Localized Charge Force Microscopy", J. Vac. Sci. Technol. A, Vol. 8, No. 1, January/February 1990, pp. 374-377. Frictional force microscopy is described in Meyer et al., "Simultaneous Measurement of Lateral and Normal Forces with an Optical-Beam-Deflection Atomic Force Microscope", Appl. Phys. Lett., Vol. 57, No. 20, Nov. 12,1990, pp. 2089-2091. As in atomic force microscopy as originally conceived by Binnig et al., the forces in all of these techniques are determined from the measured cantilever deflection and the characteristics of the cantilever. It can be argued that whereas magnetic, van der Waals, electrostatic, and frictional forces differ in magnitude and range of interaction, they are all ultimately atomic in nature. Accordingly, the term "atomic force microscope" as used herein includes any scheme in which a tip attached to a cantilever is moved with respect to a surface, and the deflection of the cantilever is used to ascertain the force exerted on the tip by the sample, regardless of the range or origin of the interaction between the tip and the sample.
AFM systems have applications beyond their original application of imaging the surface of a sample. For example, AFM systems have been proposed for data storage, as described in IBM's U.S. Pat. No. 5,537,372. In that application, the tip on the cantilever free end is in physical contact with the surface of a data storage medium. The medium has surface incongruences in the form of bumps and/or depressions that represent data. The deflection of the cantilever is detected and decoded to read the data. Data can also be written on the medium, if the medium has a heat-deformable surface, by heating the cantilever tip when it is in contact with the medium surface to form bumps or depressions on the medium surface.
In addition to tunneling current detection, several other methods of detecting the deflection of the AFM cantilever are available. Optical beam deflection is currently the most common form of detection used in commercial instruments but does not provide an integrated, purely electrical signal readout. Other methods include optical interferometry, capacitive techniques, and more recently piezoresistance.
The principle of piezoresistance to detect the deflection of the AFM cantilever is described in U.S. Pat. No. 5,345,815. The cantilever is formed of single-crystal silicon which is implanted with a dopant to provide a piezoresistive region along the length of the cantilever. Deflection of the free end of the cantilever produces stress in the cantilever. That stress changes the electrical resistance of the piezoresistive region in proportion to the cantilever's deflection. A resistance measuring apparatus is coupled to the piezoresistive region to measure its resistance and to generate a signal corresponding to the cantilever's deflection. Moving the cantilever tip across a sample for scanning is relatively straightforward with piezoresistive detection in comparison to optical detection, for which external optics must move with the cantilever. However, piezoresistive cantilevers do not have the same sensitivity as optical schemes, and also suffer from low frequency noise and temperature drift inherent in all semiconductor strain gauges. Also, they require that the cantilevers be formed of single-crystal silicon.
IBM's U.S. Pat. No. 5,345,816 describes an AFM system with strain sensors mounted on a needle-like probe on the cantilever free end to detect lateral or in-plane variations in the sample being scanned. U.S. Pat. No. 5,266,801 mentions but does not describe an embodiment of an AFM system with a strain gauge located on the cantilever for sensing deflection of the cantilever.
Giant magnetoresistance (GMR) has been observed in a variety of magnetic multilayered structures wherein the application of an external magnetic field causes a variation in the relative orientation of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes. The use of a GMR multilayered structure in the presence of an applied magnetic bias field as a strain gauge for replacement of conventional semiconductor strain gauges is described in U.S. Pat. No. 5,168,760.
A particularly useful application of GMR is a sandwich structure comprising two substantially uncoupled ferromagnetic layers separated by a nonmagnetic metallic layer in which the magnetic moment of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the layer onto an antiferromagnetic layer to exchange couple the two layers. This results in a spin valve magnetoresistive structure in which the magnetic moment of only the unpinned or free ferromagnetic layer is free to rotate in the presence of an external magnetic field. IBM's U.S. Pat. No. 5,206,590 discloses a basic spin valve magnetoresistive sensor.
It is desirable to increase the sensitivity of the cantilever detection technique in AFM systems so that smaller amounts of cantilever movement can be reliably detected.